Corrosion-resistant zinc-nickel plated bearing races

ABSTRACT

A rolling element bearing includes a first ring having a first raceway; a second ring having a second raceway, and rolling elements between the two raceways. The first and second rings are positioned and configured so that the first and second raceways form a channel which retains the rolling elements. A first zinc alloy plated layer is on the first ring including at the first raceway, and a second zinc alloy plated layer is on the second ring including at the second raceway. The rolling elements may also be plated with a zinc alloy layer. The layers are porous and thus permit hydrogen to escape from the rings and rolling elements when baked, so that the rings and rolling elements possess low hydrogen embrittlement. The layers provide physical and galvanic protection to the underlying substrates for the rings.

RELATED APPLICATION

This application is a continuation-in-part of application Ser. No.07/710,656 filed Jun. 5, 1991.

BACKGROUND OF THE INVENTION

The invention relates to corrosion resistant rolling element bearingsand a process for producing races for such bearings.

Some bearing applications require bearings which are capable of bothenduring high loads and surviving in very corrosive environments. Forexample, so-called airframe bearings, such as, the bearings on which thecontrol surfaces and flaps of aircraft oscillate, must survive exposureto moisture and deicing fluids, not to speak of salt spray on occasions.Moreover, these bearings experience wide variations in pressure whichcause them to ingest fluids, bringing those fluids into contact with theraceways which deteriorates the raceways. Bearings for machinery used inthe food processing industry likewise operate in hostile environmentscharacterized by aqueous corrosion. The high strength material fromwhich high load bearings are typically made (e.g., 52100 bearing steel)does not provide the required level of corrosion resistance for suchenvironments.

In an effort to improve the corrosion resistance of such bearings, otherbase materials, such as 316 stainless steel, have been utilized. Aproblem with many such alternative base materials, however, is that theyare not hardenable and thus are not capable of providing the requiredload handling capabilities of the high strength steels. Other stainlesssteels, such as 440C stainless, are hardenable, but do not havesufficient resistance to corrosion. Thus, insofar as the stainlesssteels are concerned, they are either corrosion resistant and incapableof acquiring suitable hardness or else capable of being hardened andincapable of resisting corrosion.

Another approach has been to deposit Thin Dense Chrome (TDC), that is, avery hard plating of chromium onto, the exposed areas including the wearor functional surfaces, such as the raceways along which the rollingelements roll. With TDC, however, it is very difficult to obtainsufficiently thick layers while still achieving the required level ofconsistency, that is, an absence of holes and surface flaws whichprovide focal points at which corrosive activity tends to occur. In thisregard, chromium is noble to steel in most corrosive environments, andthus any break in the chromium coating will cause the steel to corrodeat that break. Hence, the chromium must form a perfect physical barrier.

Yet another approach has been to deposit cadmium protective layers,which are soft in comparison to the hardened, high strength steel fromwhich the bearing is constructed. Due to its softness and othercharacteristics, cadmium is not well suited for use on the functionalsurfaces. Under load conditions, the cadmium may separate from the steelbase material and interfere with the operation of the bearing orotherwise is quickly worn off, thereby eliminating the physical andgalvanic protection which it originally provided. As a consequence, incadmium plated bearings, the cadmium does not exist along the functionalsurfaces, but instead the steel is exposed at these surfaces. Apart fromthat, the plating solution from which cadmium is derived also containscyanide which is extremely toxic. Environmental regulations do not favorcadmium plating by reason of the toxicity of the plating solution.

Most processes for plating steel rely on electrochemical reactionswithin plating solutions that contain and indeed often liberatehydrogen. During the process the steel absorbs hydrogen, and thehydrogen embrittles the steel. But a measure of ductility, notbrittleness, is desired in bearing races and the rolling elements whichmove along them. Cadmium deposits on steel in a somewhat porouscondition, and one can relieve hydrogen embrittlement simply by bakingthe steel part after it is plated. During baking the hydrogen escapesthrough the pores in the coating. Some other metals deposit on steel ina generally impervious condition and in effect trap hydrogen in thesteel so that it cannot be easily driven off by baking. Zinc andtraditional zinc alloys have exhibited this characteristic whendeposited by conventional electroplating processes.

SUMMARY OF THE INVENTION

In general, in one aspect, the invention features a rolling elementbearing including a first race having a first raceway and a second racehaving a second raceway. The first and second races are positionedrelative to each other so that the first and second raceways form achannel. The rolling element bearing also includes a first zinc alloyplated layer on the first raceway, a second zinc alloy plated layer onthe second raceway, and a plurality of rolling elements disposed withinthe channel formed by the first and second raceways.

Preferred embodiments include the following features. The rollingelement bearing also includes a lubricant (e.g. a grease) within thechannel. The first and second zinc alloys are zinc-nickel alloys. Thefirst race is an inner bearing ring and the second race is an outerbearing ring. Each of the rolling elements is a spherical bearing ball.The thickness of each of the first and second zinc alloy layers isbetween about 0.000050 and 0.000150 inch at the raceways and elsewhereis less than about 0.0010 inch, and preferably between about 0.0003 and0.0005 inch. Each of the rolling elements may include a zinc alloyplated layer on its surface, with the thickness of the zinc alloy layerbeing between about 0.000025 and 0.0001 inch.

In general, in another aspect, the invention features a rolling elementbearing including an inner ring having an outer raceway; an outer ringhaving an inner raceway, and the inner and outer rings being positionedso that the inner and outer raceways form a channel; a first zinc alloyplated layer on the inner raceway; a second zinc alloy plated layer onthe outer raceway; a plurality of bearing balls disposed within thechannel formed by the inner and outer races; and a lubricant within thechannel.

Still another aspect includes the process of providing a substrate,applying a zinc alloy layer to the substrate by an electroplatingprocess which leaves the layer with microscopic channels, and heatingthe substrate and layer to drive hydrogen from the substrate out throughthe pores in the zinc alloy layer.

One advantage of the invention is that it exhibits excellent galvaniccorrosion protection on the high load, plated functional surfaces ofrolling element bearings. Experiments have shown that even after 100hours of exposure to salt spray in accordance with ASTM B117, thefunctional surfaces or the bearing rings which have been plated withzinc-nickel showed an absence of corrosion.

In addition, the zinc-nickel plating is somewhat porous in the sensethat it contains microscopic fissures or channels which areinterconnected between the steel-plating interface and the exposed faceof the plating and allow hydrogen to escape from the steel when theplated steel is baked.

Moreover, even though the zinc-nickel plating is softer than theunderlying high strength steel from which the bearing is made, itwithstands the rolling contact under high load conditions (e.g. inexcess of 150,000 p.s.i.). In addition, it does not degrade theunderlying base material and thereby limit the load carryingcapabilities of the bearing.

Other advantages and features will become apparent from the followingdescription of the preferred embodiment and from the claims.

DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 is a cross-sectional view of a portion of a bearing assemblyhaving inner and outer rings and balls between them, all made inaccordance with and embodying the present invention;

FIG. 2 is a cross-sectional view of a portion of the outer ring;

FIG. 2A is a fragmentary expanded view, in section, of the outer ringillustrated in FIG. 2 and showing the zinc-nickel plated layer;

FIG. 3 is a cross-sectional view of a portion of the inner ring; and

FIG. 3A is a fragmentary expanded view, in section, of the inner ringillustrated in FIG. 3 and showing the zinc-nickel plated layer.

STRUCTURE AND OPERATION

FIG. 1 shows a bearing assembly 10 which will be used to illustrate theinvention. Bearing assembly 10 includes an outer race or ring 12, aninner race or ring 14 and a set of rolling elements or bearing balls 16(only one of which is shown) arranged in a single row between the tworings 12 and 14. An outer raceway 20 is formed around the insidecircumference of outer ring 12 and an inner raceway 18 is formed aroundthe outside circumference of inner ring 14. When inner ring 14 isassembled into outer ring 12, the outer and inner raceways 20 and 18 arealigned with respect to each other so as to form a channel which holdsthe set of bearing balls 16. Thus, inner ring 14 is free to rotate withrespect to the outer ring 12 or vice versa about a common axis which isperpendicular to the planes of both rings 12 and 14. The channel alsocontains a lubricant 21. That lubricant may be a grease, such as Mobil28 grease sold by Mobil Oil Corporation, or a liquid lubricant, such asoil, or even a solid lubricant, such as graphite.

As shown in greater detail in the exploded views in FIGS. 2A and 2B, azinc-nickel plated layer 22 covers outer raceway 20 and a zinc-nickelplated layer 24 covers inner raceway 18. In the described embodiment,the thickness of the plated layers 22 and 24 within the raceways isbetween approximately 0.000050 and 0.000150 inch. Zinc-nickel platedlayers 22 and 24 provide protection against corrosion which might tendto be caused by environmental conditions, e.g. the presence of saltwater.

Zinc-nickel layers 22 and 24 are applied by an electrical platingprocess as described in U.S. Pat. No. 4,765,871 issued to G. Hsu et al.on Aug. 23, 1988, and U.S. Pat. No. 4,818,632 issued to G. Hsu et al onApr. 4, 1989, both of which are incorporated herein by reference.Further information concerning the plating process and the zinc-nickelplating derived from it appears in SAE Paper 830686 entitled "A NewZinc-Nickel Electroplating Process Alternative to Cadmium Plating",Grace F. Hsu, reprinted from Proceedings of the 19th Annual AirlinePlating & Metal Finishing Forum, page 127, which paper is alsoincorporated herein by reference. In the described embodiment,zinc-nickel is electroplated onto the entire outer ring 12 and theentire inner ring 14, including the raceways 20 and 18 of those rings.During the plating process, however, inner ring 14 is oriented withinthe electroplating bath relative to the zinc and nickel anodes so as tocontrol the thickness of the plating which is formed on the outboardfaces 28 of inner ring 14 to be within the range of 0.0003 to 0.0005inch. The thickness of the plating on the raceway 18 of inner ring 14 istypically less than the controlled thickness, it being within the rangeof 0.000050 to 0.000150 inch in this region. Similarly with outer ring12, during the plating process, it is oriented within the electroplatingbath relative to the zinc and nickel anodes so as to control thethickness of the plating which is formed on the outside surface 30 andoutboard faces 33 of outer ring 12 to also be within the range of 0.0003to 0.0005 inch. As with inner ring 14 the thickness of the plating onthe raceway 20 of the outer ring 12 is also typically less than thiscontrolled thickness, it likewise being between about 0.000050 and0.000150 inch.

The plating process yields zinc-nickel layers 22 and 24 which are porousin the sense that they contain microscopic escape channels. Thesechannels extend completely through the layers 22 and 24 from theirinterfaces with the steel to the exposed exterior surfaces. Thechannels, which are described more fully in U.S. Pat. No. 4,818,632 andSAE Paper 830686, impart a porosity to the layers 22 and 24, and thisporosity enables hydrogen, which is absorbed by the steel during theplating process, to escape when the rings 12 and 14 are heated to about350° F. and held at that temperature for about 3.5 hours. Yet the escapechannels in the layers 22 and 24 are small enough to prevent significantamounts of hydrogen from reaching the steel substrate of the rings 12and 14 and reembrittling the steel when the bearing assembly 10containing the rings 12 and 14 is placed in operation.

The zinc-nickel plated layers 22 and 24 exhibit a hardness that is inthe range of about Rockwell C30 to C45, based on a conversion from amicrohardness technique of testing, which is significantly softer thanthe underlying hardened, high strength steel and significantly softerthan the thin dense chrome layers which have been used to achievecorrosion resistance on the functional surfaces of high load bearings.

Experiments have indicated that the zinc-nickel plating on raceways 18and 20 survive 120,000 cycles of 90° rotation under 400 lbs radial load,which is 20% of the maximum rated load. This is about 2.5 times thenormal design life.

Both outer ring 12 and inner ring 14 may be made of metal capable ofwithstanding stress levels of greater than 150,000 p.s.i., including forexample, thru-hardened, high strength steel (e.g., 52100 bearing steelheat treated to Rockwell C60 or higher) or case-hardened steel with itshardened region being no less than 0.025 inches thick. In the describedembodiment, rings 12 and 14 are fabricated from annealed steel in aconventional manner. The rough stock is heat treated, tempered one ormore times to improve the toughness of the steel, and then precisionground on surfaces. As an optional step, the precision ground rings maybe baked for 3.5 hours at a temperature of 350° F. prior to the platingprocess. After the optional pre-plating bake, a zinc-nickel layer iselectroplated onto rings 12 and 14, including the functional surfaces,that is, the raceways 18 and 20. To perform the plating, rings 12 and 14are rack mounted in the plating bath. They are suspended on the racks insuch a way as to not obstruct plating onto the functional surfaces.

After the plating process is complete, the rings are again baked for 3.5hours at 350° F. to avoid hydrogen embrittlement of the base metal orsubstrate. In effect, the baking drives from the steel rings 12 and 14hydrogen which they acquired during the electrochemical plating. Thehydrogen passes through the escape channels in the plated layers 22 and24 and thus is not trapped in the rings 12 and 14 by the layers 22 and24. This post-plating bake should be done within four hours of when theplating process is completed.

To extend the life of the bearings, it may also be desirable toadminister a chromate treatment to the surface of the zinc-nickel layersfollowed by another bake out. The chromate treatment prevents "whiterust" from forming on the zinc-nickel surface and postpones degradationof the zinc-nickel layers.

Other embodiments are within the following claims. For example, theinvention has applicability to the general category of rolling elementbearings, which includes among others, bearings which use taperedrollers and those which use cylindrical rollers. In addition, any one ofa broad range of zinc alloy plated deposits may be used to providesimilar corrosion resistance on the functional surfaces. Otherappropriate zinc alloys include zinc-tin, zinc-cobalt and zinc-iron, toname a few.

Also, it may be desirable to plate the rolling elements or balls 16 aswell as the races or rings 12 and 14. The plating process is essentiallythe same, but may utilize a conventional barrel plating arrangementrather than the rack mount arrangement used for plating the rings 12 and14. Due to the micro sliding which the balls 16 experience, it may bedesirable to keep the thickness of the plated layer on each ball 16 towithin the range of 0.000025 to 0.0001 inch.

In the operation of the bearing assembly 10, the outer ring 12 willrotate around the inner ring 14 or the inner ring 14 will rotate withinthe outer ring 12. In either event, the balls 16 roll within the channelformed by the raceways 18 and 20 on the inner, ring 14 and outer ring12, respectively. In other words, the balls 16 roll along the raceways18 and 20, that is to say along the layers 22 and 24 of alloy platingthat exist along the raceways 20 and 18. While the layers 24 and 22 inthe regions of the raceways 18 and 20 possess the microscopic channelsbefore the bearing assembly 10 is placed in service, thus permittingremoval of hydrogen from the underlying steel of the rings 12 and 14,the balls 10 tend to obliterate the channels during operation. In thepresence of the balls 16, the plated layers 22 and 24 undergo plasticdeformation which tends to close the microscopic channels. But this doesnot matter, since the channels have already served their purpose.Indeed, the obliteration may even be desirable for it provides smoothersurfaces along the raceways 18 and 20 and produces a further impedimentto hydrogen embrittlement from the atmosphere and to fluids encounteredin operation. Actually the layers 22 and 24 under a microscope appearsomewhat nodular everywhere, except at the raceways 20 and 18 where theyappear somewhat scuffed or smeared.

The layers 22 and 24 do not separate easily from the steel of the rings12 and 14, and if they do experience any disintegration, it is usuallyin the form of small particles. Being primarily zinc, the particles arequite malleable and hence will not damage the rings 12 and 14 or theballs 16.

To be sure, the bearing assembly 10 includes seals which close theannular spaces between the outer and inner rings 12 and 14 at the endsof the assembly 10. The seals may be fitted to the inner ring 14 andhave elastomeric lips which bear against the outer ring 12, therebyestablishing dynamic fluid barriers along the outer ring 12. But theseals may leak, particularly if the bearing assembly 10 is subjected towide variances in atmospheric pressure, as are airframe bearings. A dropin pressure will draw contaminants into the channel formed by theraceways 18 and 20, whereas a rise in pressure will purge grease fromthe channel. The layers 22 and 24, being over the steel at the raceways20 and 18, prevent the contaminants from actually contacting the steeland thus establishes a physical barrier along the raceways 20 and 22.Zinc, being higher than iron in the electromotive-force series forpractically all corrosive environments further enables the plated layers22 and 24 to provide galvanic protection for the steel of the rings 12and 14.

Beyond the region enclosed by the seals, the surfaces 30 and 33 of theouter ring 12 and the faces 28 of the inner ring 14 are exposed, and therings 12 and 14 would quickly corrode in these areas were it not for theplated layers 22 and 24 which exist along them. The layers 22 and 24 inthese regions prevent the steel of the rings 12 and 14 from coming intocontact with the atmosphere and any fluids to which the bearing assembly10 may be subjected, such as deicing solution, salt spray, or simplywater. Even if the physical barrier formed by the layers 22 and 24 isdisrupted by reason of being damaged, the underlying steel of the rings12 and 14 will not corrode owing to the galvanic protection provided bythe zinc of the layers. Likewise, the presence of the fissures ormicroscopic channels in the layers 22 and 24 does not result in anycorrosion of the underlying steel in the rings 12 and 14. Thus, thelayers 22 and 24 provide a high level of physical and galvanicprotection for the steel substrates of the rings 12 and 14.

What is claimed is:
 1. A rolling element bearing comprising:a first racehaving a first raceway; a second race having a second raceway, saidfirst and second races being positioned so that said first and secondraceways form a channel; a first zinc alloy plated layer on said firstrace along the first raceway thereof; a second zinc alloy plated layeron said second race along the second raceway thereof; and a plurality ofrolling elements disposed within said channel formed by said first andsecond raceways.
 2. The rolling element bearing of claim 1 furthercomprising a lubricant within said channel.
 3. The rolling elementbearing of claim 2 wherein said lubricant is a grease.
 4. The rollingelement bearing of claim 2 wherein said first race is an inner bearingring and said second element is an outer bearing ring.
 5. The rollingelement bearing of claim 4 wherein each of said rolling elements isspherical.
 6. The rolling element bearing of claim 1 wherein zinc alloyof the plated layers is porous and the first and second races have lowhydrogen embrittlement by reason of hydrogen formerly contained withinthe races having escaped through the pores in the layers of those races.7. The rolling element bearing of claim 1 wherein said first and secondzinc alloys are zinc-nickel alloys.
 8. The rolling element bearing ofclaim 1 wherein the thickness of each of the first and second zinc alloylayers is less than about 0.0010 inch.
 9. The rolling element bearing ofclaim 8 wherein the thickness of each of the first and second zinc alloylayers is between about 0.0003 and 0.0005 inches remote from theraceways.
 10. The rolling element bearing of claim 1 wherein each ofsaid plurality of rolling elements includes a zinc alloy plated layer onits surface.
 11. The rolling element bearing of claim 10 wherein thezinc alloy plated layer on each of said plurality of rolling elements isless than about 0.0001 inch.
 12. An antifriction bearing comprising:afirst race having a raceway; a second race having a raceway that ispresented toward the raceway of the first race; at least one of theraces including a substrate and zinc alloy over the substrate in theform of a plating, the zinc alloy plating being at the raceway of thefirst race and elsewhere on the first race as well, the zinc alloyplating having microscopic pores which extend through it from thesubstrate to its exposed surface; and rolling elements located betweenthe races at the raceways thereof.
 13. A bearing according to claim 12wherein substrate of said one race is steel having low hydrogenembrittlement by reason of having been heated to the extent thathydrogen formerly contained within it has been driven off through thepores in the zinc alloy layer.
 14. A bearing according to claim 12wherein each race has a substrate of steel and a zinc alloy plating overthe substrate, and the steel substrate has low hydrogen embrittlement.15. A bearing according to claim 14 wherein the zinc alloy plating oneach of the races is applied electrically and also includes nickel. 16.A bearing according to claim 15 wherein the zinc alloy plating on eachof the races covers substantially the entire race.
 17. A bearingaccording to claim 14 wherein each rolling element includes a steelsubstrate and a zinc alloy plating over the substrate.
 18. A bearingaccording to claim 17 wherein the zinc alloy plating on the rollingelements includes nickel.
 19. A bearing according to claim 12 whereinthe zinc alloy plating of said one race covers substantially the entirerace.
 20. A bearing according to claim 12 wherein the zinc alloy layeralso contains nickel. .Iadd.
 21. An antifriction bearing comprising:afirst race having a raceway; a second race having a raceway that ispresented toward the raceway of the first race; at least one of theraces including a substrate and zinc alloy over the substrate in theform of a plating, the zinc alloy plating being at least along theraceway of said one race; and rolling elements located between the racesat the raceways thereof. .Iaddend..Iadd.22. A bearing according to claim21 wherein the zinc alloy plating of said one race covers substantiallythe entire race. .Iaddend..Iadd.23. A bearing according to claim 21wherein the zinc alloy plating also contains nickel. .Iaddend.